Tabs

In the context of buliding the prototype for http://github.com/GEMScienceTools/hmtk this pull request investigates the possibility of creating plugins for the GUI aimed at adding new methods to hmtk. More precisely, it implements

0) A minimal plugin system for loading external python modules
1) Dynamic form creation. Each group of hmtk methods is managed by a `Tab` instance. In each tab the user, upon method selection, is presented a form tailored for the method with proper widgets for each given input type. The form structure is derived by the informations inspected at runtime on the method implementation class. Requires https://github.com/GEMScienceTools/hmtk/pull/22
2) A Map "info" tool the user can use to inspect the catalogue layer and the raster one (if available).